1. Technical Field
A method for fabricating a semiconductor device and, more particularly, a method for stripping photoresist material and cleaning after via contact etching is disclosed.
2. Description of the Related Art
As the density and performance of current VLSI (Very Large Scale Integration) increases, the significance of a multi-layer metal interconnection line structure increases. A multi-layer structure is formed from a stack of metal interconnection lines and inter-layer insulation layers separating the interconnecting lines. For high performance of a semiconductor device, reduction of RC (resistor-capacitor) delay factors in the interconnection line structure is required. So, for metal interconnection lines, it is necessary to reduce the resistance values, and therefore copper (Cu) is used because its inherent resistance value is less than that of aluminum (Al). Whereas for insulation layers, an insulation layer with a low dielectric constant (low-k) is needed to reduce the capacity between the interconnection lines and between the layers.
Meanwhile, as the structure of interconnection lines becomes smaller with more layers, the via contact holes electrically connecting the interconnection lines become smaller while the via contact resistance increases. Via contact holes are formed after going through the mask process of depositing and developing a photoresist, and performing via contact etching.
Via contact etching is different from other contact etchings in that the lower layer of the insulation layer is metal. In other words, because the lower layer is metal, metal is sputtered physically while etching the insulation layer, and the sputtered metal atoms stick to the sidewalls or bottom of a contact hole and forms a residue. Containing many metallic components, the residue on the side and bottom of the via contact can be difficult to remove.
FIG. 1 is a cross-sectional view showing a semiconductor device after conventional via contact etching. An inter-metal insulation layer 105 is molded on top of metal lower interconnection lines 100 and a photoresist pattern is formed for via contact etching. A via contact hole is molded by etching the inter-metal insulation layer 105 with the photoresist pattern 120 being used as an etching barrier. Then, the photoresist pattern 120 is formed of two photoresists: one is a photoresist 110 which was not deformed during the etching procedure and the other is a photoresist 115 deformed by fluorine-based gas used for stripping the photoresist. Also, residue 125 containing metallic components is formed on the bottom and the sidewalls of the via holes.
Therefore, after via contact etching, the photoresist, residue and metal contamination all should be removed by performing a cleaning procedure. The conventional cleaning procedures after via contact etching are conducted in by stripping the photoresist, followed by wet-cleaning using ACT™ solvent and scrubbing before depositing a metal. The conventional method of stripping the photoresist and cleaning method will be described hereinafter.
First of all, the photoresist used as the barrier for via hole etching needs to be removed because it does not perform any function. To get rid of a photoresist, which consist of organic substances, a method of “burning”, that is, a method reacting the photoresist with oxygen is commonly used. In other words, the photoresist resolves in an oxygen plasma as an oxygen atom excited in the plasma reacts to carbon in the photoresist and becomes a carbon dioxide.
However, photoresist polymer materials have a complicated chemical structure, so the actual oxygen resolution reaction is not straight forward and, often, is not complete. Also, in case there are non-volatile substances among the oxide substances of the components of the photoresist, they may be oxidized and became a residue during the ashing procedure. As seen above, it is not easy to remove a photoresist deformed by ions or etching gases, and as the integration of a semiconductor device gets higher, if the size of the interconnection lines shrinks, the remaining bit of a photoresist may trigger a defect and be a culprit for disconnection and short circuit, thereby dropping the throughput.
Meanwhile, the gas added to oxygen gas during the photoresist strip has been under examination. To improve the ashing rate and exfoliation, fluorine-based reactive gases, and hydrogen-based reactive gases or a mixture of gases including hydrogen are used in addition to the oxygen gas as common process gases.
Subsequently, procedures including ACT wet cleaning and scrubbing are carried out to remove the residue stuck on the bottom and the sidewalls after stripping the photoresist. Recently, however, wet cleaning exhibits a problem in supplying ACT and it is not an economical process to perform. Also, in case of forming the inter-metal insulation layer 105 with a low-k insulation layer, the insulation layer may be attacked while being cleaned and thus the contact holes may have a bowing phenomenon. This will affect the step coverage in the subsequent procedures of metal deposition, causing a problem of reducing throughput.